Unit-3

If statements

The ability to control the flow of your program, letting it make decisions on what code to execute, is valuable to the programmer. The if statement allows you to control if a program enters a section of code or not based on whether a given condition is true or false. One of the important functions of the if statement is that it allows the program to select an action based upon the user's input. For example, by using an if statement to check a user-entered password, your program can decide whether a user is allowed access to the program.

Without a conditional statement such as the if statement, programs would run almost the exact same way every time, always following the same sequence of function calls. If statements allow the flow of the program to be changed, which leads to more interesting code.

Before discussing the actual structure of the if statement, let us examine the meaning of TRUE and FALSE in computer terminology. A true statement is one that evaluates to a nonzero number. A false statement evaluates to zero. When you perform comparison with the relational operators, the operator will return 1 if the comparison is true, or 0 if the comparison is false. For example, the check 0 == 2 evaluates to 0. The check 2 == 2 evaluates to a 1. If this confuses you, try to use a printf statement to output the result of those various comparisons (for example printf ( "%d", 2 == 1 );)

When programming, the aim of the program will often require the checking of one value stored by a variable against another value to determine whether one is larger, smaller, or equal to the other.

There are a number of operators that allow these checks.

Here are the relational operators, as they are known, along with examples:

 
> greater than 5 > 4 is TRUE
< less than 4 < 5 is TRUE
>= greater than or equal 4 >= 4 is TRUE
<= less than or equal 3 <= 4 is TRUE
== equal to 5 == 5 is TRUE
!= not equal to 5 != 4 is TRUE

It is highly probable that you have seen these before, probably with slightly different symbols. They should not present any hindrance to understanding. Now that you understand TRUE and FALSE well as the comparison operators, let us look at the actual structure of if statements.

The structure of an if statement is as follows:

 
if ( statement is TRUE )
 Execute this line of code

Here is a simple example that shows the syntax:

 
if ( 5 < 10 )
 printf( "Five is now less than ten, that's a big surprise" );

Here, we're just evaluating the statement, "is five less than ten", to see if it is true or not; with any luck, it's not! If you want, you can write your own full program including stdio.h and put this in the main function and run it to test.

To have more than one statement execute after an if statement that evaluates to true, use braces, like we did with the body of the main function. Anything inside braces is called a compound statement, or a block. When using if statements, the code that depends on the if statement is called the "body" of the if statement.

For example:

 
if ( TRUE ) {
 /* between the braces is the body of the if statement */
 Execute all statements inside the body
}

I recommend always putting braces following if statements. If you do this, you never have to remember to put them in when you want more than one statement to be executed, and you make the body of the if statement more visually clear.

Else

Sometimes when the condition in an if statement evaluates to false, it would be nice to execute some code instead of the code executed when the statement evalutes to true. The "else" statement effectively says that whatever code after it (whether a single line or code between brackets) is executed if the if statement is FALSE.

It can look like this:

 
if ( TRUE ) {
 /* Execute these statements if TRUE */
}
else {
 /* Execute these statements if FALSE */
}

Else if

Another use of else is when there are multiple conditional statements that may all evaluate to true, yet you want only one if statement's body to execute. You can use an "else if" statement following an if statement and its body; that way, if the first statement is true, the "else if" will be ignored, but if the if statement is false, it will then check the condition for the else if statement. If the if statement was true the else statement will not be checked. It is possible to use numerous else if statements to ensure that only one block of code is executed.

Let's look at a simple program for you to try out on your own.

 
#include <stdio.h> 
 
int main() /* Most important part of the program!
*/
{
 int age; /* Need a variable... */
 
 printf( "Please enter your age" ); /* Asks for age */
 scanf( "%d", &age );  /* The input is put in age */
 if ( age < 100 ) { /* If the age is less than 100 */
 printf ("You are pretty young!\n" ); /* Just to show you it works... */
 }
 else if ( age == 100 ) { /* I use else just to show an example */ 
 printf( "You are old\n" ); 
 }
 else {
 printf( "You are really old\n" ); /* Executed if no other statement is
 */
 }
 return 0;
}

More interesting conditions using boolean operators

Boolean operators allow you to create more complex conditional statements. For example, if you wish to check if a variable is both greater than five and less than ten, you could use the Boolean AND to ensure both var > 5 and var < 10 are true. In the following discussion of Boolean operators, I will capitalize the Boolean operators in order to distinguish them from normal English. The actual C operators of equivalent function will be described further along into the tutorial - the C symbols are not: OR, AND, NOT, although they are of equivalent function.

When using if statements, you will often wish to check multiple different conditions. You must understand the Boolean operators OR, NOT, and AND. The boolean operators function in a similar way to the comparison operators: each returns 0 if evaluates to FALSE or 1 if it evaluates to TRUE.

NOT: The NOT operator accepts one input. If that input is TRUE, it returns FALSE, and if that input is FALSE, it returns TRUE. For example, NOT (1) evalutes to 0, and NOT (0) evalutes to 1. NOT (any number but zero) evaluates to 0. In C NOT is written as !. NOT is evaluated prior to both AND and OR.

AND: This is another important command. AND returns TRUE if both inputs are TRUE (if 'this' AND 'that' are true). (1) AND (0) would evaluate to zero because one of the inputs is false (both must be TRUE for it to evaluate to TRUE). (1) AND (1) evaluates to 1. (any number but 0) AND (0) evaluates to 0. The AND operator is written && in C. Do not be confused by thinking it checks equality between numbers: it does not. Keep in mind that the AND operator is evaluated before the OR operator.

OR: Very useful is the OR statement! If either (or both) of the two values it checks are TRUE then it returns TRUE. For example, (1) OR (0) evaluates to 1. (0) OR (0) evaluates to 0. The OR is written as || in C. Those are the pipe characters. On your keyboard, they may look like a stretched colon. On my computer the pipe shares its key with \. Keep in mind that OR will be evaluated after AND.

It is possible to combine several Boolean operators in a single statement; often you will find doing so to be of great value when creating complex expressions for if statements. What is !(1 && 0)? Of course, it would be TRUE. It is true is because 1 && 0 evaluates to 0 and !0 evaluates to TRUE (ie, 1).

Try some of these - they're not too hard. If you have questions about them, feel free to stop by our forums.

 
A. !( 1 || 0 ) ANSWER: 0 
B. !( 1 || 1 && 0 ) ANSWER: 0 (AND is evaluated before OR)
C. !( ( 1 || 0 ) && 0 ) ANSWER: 1 (Parenthesis are useful)

If you find you enjoyed this section, then you might want to look more at Boolean Algebra.

Loops

Loops are used to repeat a block of code. Being able to have your program repeatedly execute a block of code is one of the most basic but useful tasks in programming -- many programs or websites that produce extremely complex output (such as a message board) are really only executing a single task many times. (They may be executing a small number of tasks, but in principle, to produce a list of messages only requires repeating the operation of reading in some data and displaying it.) Now, think about what this means: a loop lets you write a very simple statement to produce a significantly greater result simply by repetition.

One caveat: before going further, you should understand the concept of C's true and false, because it will be necessary when working with loops (the conditions are the same as with if statements). This concept is covered in the previous tutorial. There are three types of loops: for, while, and do..while. Each of them has their specific uses. They are all outlined below.

FOR - for loops are the most useful type. The syntax for a for loop is

 
for ( variable initialization; condition; variable update ) {
 Code to execute while the condition is true
}

The variable initialization allows you to either declare a variable and give it a value or give a value to an already existing variable. Second, the condition tells the program that while the conditional expression is true the loop should continue to repeat itself. The variable update section is the easiest way for a for loop to handle changing of the variable. It is possible to do things like x++, x = x + 10, or even x = random ( 5 ), and if you really wanted to, you could call other functions that do nothing to the variable but still have a useful effect on the code. Notice that a semicolon separates each of these sections, that is important. Also note that every single one of the sections may be empty, though the semicolons still have to be there. If the condition is empty, it is evaluated as true and the loop will repeat until something else stops it.

Example:

 
#include <stdio.h>
 
int main()
{
 int x;
 /* The loop goes while x < 10, and x increases by one every loop*/
 for ( x = 0; x < 10; x++ ) {
 /* Keep in mind that the loop condition checks 
 the conditional statement before it loops again.
 consequently, when x equals 10 the loop breaks.
 x is updated before the condition is checked. */ 
 printf( "%d\n", x );
 }
 getchar();
}

This program is a very simple example of a for loop. x is set to zero, while x is less than 10 it calls printf to display the value of the variable x, and it adds 1 to x until the condition is met. Keep in mind also that the variable is incremented after the code in the loop is run for the first time.

WHILE - WHILE loops are very simple. The basic structure is

while ( condition ) { Code to execute while the condition is true }

The true represents a boolean expression which could be x == 1 or while ( x != 7 ) (x does not equal 7). It can be any combination of boolean statements that are legal. Even, (while x ==5 || v == 7) which says execute the code while x equals five or while v equals 7. Notice that a while loop is like a stripped-down version of a for loop-- it has no initialization or update section. However, an empty condition is not legal for a while loop as it is with a for loop.

Example:

 
#include <stdio.h>
 
int main()
{ 
 int x = 0; /* Don't forget to declare variables */
 
 while ( x < 10 ) { /* While x is less than 10 */
 printf( "%d\n", x );
 x++; /* Update x so the condition can be met eventually */
 }
 getchar();
}

This was another simple example, but it is longer than the above FOR loop. The easiest way to think of the loop is that when it reaches the brace at the end it jumps back up to the beginning of the loop, which checks the condition again and decides whether to repeat the block another time, or stop and move to the next statement after the block.

DO..WHILE - DO..WHILE loops are useful for things that want to loop at least once. The structure is

 
do {
} while ( condition );

Notice that the condition is tested at the end of the block instead of the beginning, so the block will be executed at least once. If the condition is true, we jump back to the beginning of the block and execute it again. A do..while loop is almost the same as a while loop except that the loop body is guaranteed to execute at least once. A while loop says "Loop while the condition is true, and execute this block of code", a do..while loop says "Execute this block of code, and then continue to loop while the condition is true".

Example:

 
#include <stdio.h>
 
int main()
{
 int x;
 
 x = 0;
 do {
 /* "Hello, world!" is printed at least one time
 even though the condition is false*/
 printf( "%d\n", x );
 } while ( x != 0 );
 getchar();
}

Keep in mind that you must include a trailing semi-colon after the while in the above example. A common error is to forget that a do..while loop must be terminated with a semicolon (the other loops should not be terminated with a semicolon, adding to the confusion). Notice that this loop will execute once, because it automatically executes before checking the condition.

Break and Continue

Two keywords that are very important to looping are break and continue. The break command will exit the most immediately surrounding loop regardless of what the conditions of the loop are. Break is useful if we want to exit a loop under special circumstances. For example, let's say the program we're working on is a two-person checkers game. The basic structure of the program might look like this:

while (true) 
{
 take_turn(player1);
 take_turn(player2);
}

This will make the game alternate between having player 1 and player 2 take turns. The only problem with this logic is that there's no way to exit the game; the loop will run forever! Let's try something like this instead:

while(true)
{
 if (someone_has_won() || someone_wants_to_quit() == TRUE)
 {break;}
 take_turn(player1);
 if (someone_has_won() || someone_wants_to_quit() == TRUE)
 {break;}
 take_turn(player2);
}

This code accomplishes what we want--the primary loop of the game will continue under normal circumstances, but under a special condition (winning or exiting) the flow will stop and our program will do something else.
Continue is another keyword that controls the flow of loops. If you are executing a loop and hit a continue statement, the loop will stop its current iteration, update itself (in the case of for loops) and begin to execute again from the top. Essentially, the continue statement is saying "this iteration of the loop is done, let's continue with the loop without executing whatever code comes after me." Let's say we're implementing a game of Monopoly. Like above, we want to use a loop to control whose turn it is, but controlling turns is a bit more complicated in Monopoly than in checkers. The basic structure of our code might then look something like this:

for (player = 1; someone_has_won == FALSE; player++)
 {
 if (player > total_number_of_players)
 {player = 1;}
 if (is_bankrupt(player))
 {continue;}
 take_turn(player);
 }

This way, if one player can't take her turn, the game doesn't stop for everybody; we just skip her and keep going with the next player's turn.

Functions

Now that you should have learned about variables, loops, and conditional statements it is time to learn about functions. You should have an idea of their uses as we have already used them and defined one in the guise of main. Getchar is another example of a function. In general, functions are blocks of code that perform a number of pre-defined commands to accomplish something productive. You can either use the built-in library functions or you can create your own functions.

Functions that a programmer writes will generally require a prototype. Just like a blueprint, the prototype gives basic structural information: it tells the compiler what the function will return, what the function will be called, as well as what arguments the function can be passed. When I say that the function returns a value, I mean that the function can be used in the same manner as a variable would be. For example, a variable can be set equal to a function that returns a value between zero and four.

For example:

 
#include <stdlib.h> /* Include rand() */
 
int a = rand(); /* rand is a standard function that all compilers have */

Do not think that 'a' will change at random, it will be set to the value returned when the function is called, but it will not change again.

The general format for a prototype is simple:

 
return-type function_name ( arg_type arg1, ..., arg_type argN ); 

arg_type just means the type for each argument -- for instance, an int, a float, or a char. It's exactly the same thing as what you would put if you were declaring a variable.

There can be more than one argument passed to a function or none at all (where the parentheses are empty), and it does not have to return a value. Functions that do not return values have a return type of void. Let's look at a function prototype:

 
int mult ( int x, int y );

This prototype specifies that the function mult will accept two arguments, both integers, and that it will return an integer. Do not forget the trailing semi-colon. Without it, the compiler will probably think that you are trying to write the actual definition of the function.

When the programmer actually defines the function, it will begin with the prototype, minus the semi-colon. Then there should always be a block (surrounded by curly braces) with the code that the function is to execute, just as you would write it for the main function. Any of the arguments passed to the function can be used as if they were declared in the block. Finally, end it all with a cherry and a closing brace. Okay, maybe not a cherry.

Let's look at an example program:

 
#include <stdio.h>
 
int mult ( int x, int y );
 
int main()
{
 int x;
 int y;
 
 printf( "Please input two numbers to be multiplied: " );
 scanf( "%d", &x );
 scanf( "%d", &y );
 printf( "The product of your two numbers is %d\n", mult( x, y ) );
 getchar(); 
}
 
int mult (int x, int y)
{
 return x * y;
}

This program begins with the only necessary include file. Next is the prototype of the function. Notice that it has the final semi-colon! The main function returns an integer, which you should always have to conform to the standard. You should not have trouble understanding the input and output functions if you've followed the previous tutorials.

Notice how printf actually takes the value of what appears to be the mult function. What is really happening is printf is accepting the value returned by mult, not mult itself. The result would be the same as if we had use this print instead

 
printf( "The product of your two numbers is %d\n", x * y );

The mult function is actually defined below main. Because its prototype is above main, the compiler still recognizes it as being declared, and so the compiler will not give an error about mult being undeclared. As long as the prototype is present, a function can be used even if there is no definition. However, the code cannot be run without a definition even though it will compile.

Prototypes are declarations of the function, but they are only necessary to alert the compiler about the existence of a function if we don't want to go ahead and fully define the function. If mult were defined before it is used, we could do away with the prototype--the definition basically acts as a prototype as well.

Return is the keyword used to force the function to return a value. Note that it is possible to have a function that returns no value. If a function returns void, the retun statement is valid, but only if it does not have an expression. In otherwords, for a function that returns void, the statement "return;" is legal, but usually redundant. (It can be used to exit the function before the end of the function.)

The most important functional (pun semi-intended) question is why do we need a function? Functions have many uses. For example, a programmer may have a block of code that he has repeated forty times throughout the program. A function to execute that code would save a great deal of space, and it would also make the program more readable. Also, having only one copy of the code makes it easier to make changes. Would you rather make forty little changes scattered all throughout a potentially large program, or one change to the function body? So would I.

Another reason for functions is to break down a complex program into logical parts. For example, take a menu program that runs complex code when a menu choice is selected. The program would probably best be served by making functions for each of the actual menu choices, and then breaking down the complex tasks into smaller, more manageable tasks, which could be in their own functions. In this way, a program can be designed that makes sense when read. And has a structure that is easier to understand quickly. The worst programs usually only have the required function, main, and fill it with pages of jumbled code.

switch case

Switch case statements are a substitute for long if statements that compare a variable to several "integral" values ("integral" values are simply values that can be expressed as an integer, such as the value of a char). The basic format for using switch case is outlined below. The value of the variable given into switch is compared to the value following each of the cases, and when one value matches the value of the variable, the computer continues executing the program from that point.

 
switch ( <variable> ) {
case this-value:
 Code to execute if <variable> == this-value
 break;
case that-value:
 Code to execute if <variable> == that-value
 break;
...
default:
 Code to execute if <variable> does not equal the value following any of the cases
 break;
}

The condition of a switch statement is a value. The case says that if it has the value of whatever is after that case then do whatever follows the colon. The break is used to break out of the case statements. Break is a keyword that breaks out of the code block, usually surrounded by braces, which it is in. In this case, break prevents the program from falling through and executing the code in all the other case statements. An important thing to note about the switch statement is that the case values may only be constant integral expressions. Sadly, it isn't legal to use case like this:

 
int a = 10;
int b = 10;
int c = 20;
 
switch ( a ) {
case b:
 /* Code */
 break;
case c:
 /* Code */
 break;
default:
 /* Code */
 break;
}

The default case is optional, but it is wise to include it as it handles any unexpected cases. It can be useful to put some kind of output to alert you to the code entering the default case if you don't expect it to. Switch statements serve as a simple way to write long if statements when the requirements are met. Often it can be used to process input from a user.

Below is a sample program, in which not all of the proper functions are actually declared, but which shows how one would use switch in a program.

 
#include <stdio.h>
 
void playgame();
void loadgame();
void playmultiplayer();
 
int main()
{
 int input;
 
 printf( "1. Play game\n" );
 printf( "2. Load game\n" );
 printf( "3. Play multiplayer\n" );
 printf( "4. Exit\n" );
 printf( "Selection: " );
 scanf( "%d", &input );
 switch ( input ) {
 case 1: /* Note the colon, not a semicolon */
 playgame();
 break;
 case 2: 
 loadgame();
 break;
 case 3:  
 playmultiplayer();
 break;
 case 4: 
 printf( "Thanks for playing!\n" );
 break;
 default: 
 printf( "Bad input, quitting!\n" );
 break;
 }
 getchar();
 
}

This program will compile, but cannot be run until the undefined functions are given bodies, but it serves as a model (albeit simple) for processing input. If you do not understand this then try mentally putting in if statements for the case statements. Default simply skips out of the switch case construction and allows the program to terminate naturally. If you do not like that, then you can make a loop around the whole thing to have it wait for valid input. You could easily make a few small functions if you wish to test the code.